Most of knowledge about Strouhal instability and Von Karman Vortex Street is experimental. Essential basics are exposed in this page.
I - Reynolds number
II - Flow patterns
III - Von Karman Vortex Street
The classical case studied is a cylinder in a laminar flow. Several streams are observable,
depending on the Reynolds number. It is defined as:
Depending on the Reynolds, experimental observations show various type of flow pattern described below.
|Re<<1||In first case, for the lower Reynolds, the flow is symmetrical upstream and downstream.|
|Re<4||The symmetry disappears for higher Reynolds and the flow is disturbed in a much longer distance behind the cylinder.|
|4<Re<40||Re-circulations appear in the wake behind the cylinder for Reynolds about 4. They develop with Reynolds and oscillate for higher Reynolds, creating oscillations in the wake.|
|40<Re<400||Up to critical Reynolds, about 40, oscillations are so strong that one of the two vortices break away from the cylinder. This is Strouhal instability. The second vortex is shed while the first one is shaped again. The vortices appear and are shed alternatively at a constant frequency. About Reynolds 200, the structure of Von Karman Vortex Street becomes three-dimensional.|
|400<Re||When Reynolds is upper 400, turbulence develop and Von Karman Vortex Street disappears.|
The frequency with which vortices are shed behind a circular cylinder in a Karman vortex street is called "n". Experimental measurements show that the dimensionless frequency, called Strouhal number
When a region with an adverse pressure gradient exists along the wall, the retarded fluid particles cannot penetrate too far into the region of increased pressure owing to their small kinetic energy. Thus, the boundary layer is deflected sideways from the wall, separates from it, and moves into the main stream : this phenomenon is called separation. The fluid particles behind the point of separation follow the pressure gradient and move in a direction opposite to the external stream.
The point of separation is defined as the limit between forward and reverse flow in the boundary layer in the immediate neighborhood of the wall:
A short distance downstream from the point of separation, the boundary layer becomes so thick that a wake is created
Velocity distribution near the point of separation